Bidirectional Integrated CMOS Switch

ABSTRACT

A bidirectional integrated CMOS switch is provided which is capable of switching voltages beyond the range of the supply and ground potentials. The switch is composed of NMOS and PMOS transistors as the switch conductor path, a diode bridge, and control circuitry to turn the switch on and off by means of low voltage logic, regardless of the voltages on the switch terminals. The device and method of the invention enables the switching of high voltage loads operating at arbitrary or floating voltages relative to the low voltage power supply and ground, and provides on/off control of the switch with ordinary low voltage logic levels. The invention provides bidirectional switching without conducting through the parasitic body diodes of the CMOS devices.

The present invention claims priority on U.S. Provisional PatentApplication Ser. No. 61/902,910, filed Nov. 12, 2013, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

CMOS is the dominant technology in almost all VLSI design. CMOS circuitsuse a combination of p-channel (PMOS) and n-channel (NMOS) metal oxidesemiconductor field effect transistors (MOSFETs) to implement logicgates and switches. The composition of a PMOS transistor creates lowresistance between its source and drain contacts when a low gate voltageis applied and high resistance when a high gate voltage is applied. Onthe other hand, the composition of an NMOS transistor creates highresistance between source and drain when a low gate voltage is appliedand low resistance when a high gate voltage is applied. A CMOS logicinverter accomplishes current reduction by complementing every NMOStransistor with a PMOS transistor and connecting both gates and bothdrains together.

CMOS transmission gates, also known as CMOS analog switches (herein“CMOS switch(es)”) often form the interface between analog signals anddigital controllers. The internal construction of a typical CMOS switchconnects an NMOS in parallel with a PMOS to enable signals to pass ineither direction with equal ease. The MOSFETs' sources are connected toeach other and their drains are connected to each other. The terminalsof the switched conduction path are the sources on one side and thedrains on the other side. The switch is controlled by the gate-sourcevoltages of the NMOS and PMOS devices. Whether the n- or p-channeldevice carries more signal current depends on the ratio of input tooutput voltage. Because the switch has no preferred direction forcurrent flow, it has no preferred input or output. The typical CMOSswitch can only be used if the terminal voltages are within the range ofthe supply rails.

CMOS switches, especially when used in power applications, often do notget the proper voltage values at their switch terminals to open andclose correctly because they are often integrated on VLSI chips withdense digital and analog circuitry. A CMOS device requires control ofthe gate-to-source voltage to keep it within the maximum limits set bythe process specifications which are typically much lower than themaximum limits for drain to source voltage. For example, a high-voltageCMOS switch device may have a drain-source voltage rating of 100 volts,but the maximum gate-source voltage may be only 5 volts. When the CMOSsource voltage is not within the range of the supply voltage, some meansmust be provided to generate the gate voltage necessary to turn on thedevice. It is also necessary to hold the gate-source voltage to zerowhen the switch is in the off state. The standard practice in the art isto use charge pumps or level shifters that are used to provide the gatedrive for CMOS switches in high voltage applications. The use of chargepumps, level shifters, or similar techniques typically do not providegate voltage control independent of the polarity of the switch terminalvoltages relative to each other and relative to the low voltage powersupply rails.

CMOS devices are prone to a parasitic effect known as the parasiticdiode of MOSFET, more commonly described as “the body diode”, whichcauses an unwanted flow/direction of current over the body of the CMOSdevice, rather than the channel. The body diode is a consequence of thehigh volume CMOS design. The body diode occurs when a diode forms in thebody, source and drain regions. When it is forward biased, it becomes analternative path for current to flow and most of the current may pass tothe body instead of through the channel. When reversed biased, it willdevelop capacitance due to the inherent nature of the diode. Theformation of a parasitic diode (herein “body diode”) can cause alatch-up or circuit failure.

Body diodes occur when using standard CMOS switches. The body diode ofthe PMOS device will conduct if the voltage on either the source ordrain is greater than the positive supply voltage. Likewise, the bodydiode of the NMOS device will conduct if the voltage on either thesource or Drain is less than the negative supply voltage. This is thereason why a standard CMOS switch can only be used if the source anddrain terminal voltages are within the range of the supply rails. In anon-isolated CMOS process, the body of the NMOS device is connected to acommon P-type silicon substrate. The source and drain of the NMOS areN-type implants on top of the P substrate. The body diodes are formed bythe P-N junctions from body-to-drain and body-to-source. The body of thePMOS device is an N-well located on top of the common P-type substrate.In a non-isolated process without SOI and trench isolation, the Psubstrate is connected to the negative supply rail. Thus, the body ofthe NMOS device is always connected to the negative supply. Thesubstrate underneath the N-well body of the PMOS device is the samesubstrate, connected to the negative supply. It is not possible to allowthe voltage of the N-well body of the PMOS device to drop below thenegative supply voltage without causing the substrate diode to conduct.

There are non-Silicon-On-Insulator processes that advertise isolateddevices using a deep Nwell or N buried layer (NBL) that runs under theP-type body of the NMOS device and around the sides. In that case, thebody of the NMOS device can be connected to a voltage other than thenegative supply voltage, provided that the deep N-well is always biasedwith a voltage greater than or equal to the body voltage so as to avoidconduction through the P-N junction diode formed by the P body and deepN-well. The deep N-well also has to be biased at a voltage greater thanor equal to the negative supply rail to avoid conduction through thediode formed by the P substrate and deep Nwell. The structure of thePMOS device is typically the same as in a non-isolated process. All ofthose parasitic paths place restrictions on the voltages that can beapplied to the deep N and to the CMOS body, drain and source terminals.Hence, there is a need for a bidirectional integrated CMOS switchcapable of switching voltages beyond the range of supply and groundpotentials. Additionally, there is a need for a means to preventconduction through the parasitic body diodes of the CMOS switch. Thepresent invention enables switching high voltage loads operating atarbitrary or floating voltages relative to the low voltage power supplyand ground, and provides on/off control of the switch with ordinary lowvoltage logic levels. The present invention can operate with highterminal voltages above and below the supply rails, which standard CMOStransmission gates cannot do. The present invention provides the propergate voltages necessary to operate the switch, regardless of the switchterminal voltages. The present invention provides bidirectionalswitching without conducting through the parasitic body diodes of theCMOS devices. The present invention can be integrated on the same CMOSchip as other circuitry, eliminating the need for separate TRIALswitches, optocouplers, or other external devices. No charge pumps orhigh voltage supplies are needed. The present invention provides costefficiencies because of its efficient integration with other componentsand its lower failure rate.

DETAILED DESCRIPTION OF THE PRIOR ART

Switches currently exist in the art that are designed to operate beyondthe supply rail voltages. One such device is theMAX14777—Quad-Beyond-the-Rails −15V to +35V Analog Switch, offered byMaxim Integrated Products Inc. This device is distinguished from thepresent invention because the Maxim device uses charge pumps to developa secondary set of high voltage positive and negative supply rails toaccomplish the switching above and below the rails. The specificationfor the MAX14777, available athttp://datasheets.maximintegrated.com/en/ds/MAX14777.pdf discloses thatthe device contains a total of three charge pumps to generate biasvoltages for the internal switches. Conversely, the present inventioneliminates the need to use charge pumps.

U.S. Pat. No. 8,461,905 discloses an adaptive bootstrap circuit forcontrolling CMOS switches. It is distinguished from the presentinvention as it (a) does not prevent conduction through the parasiticbody diodes of the CMOS device; (b) does not accommodate switch drainand source voltages beyond the range of the supply rails in the ONstate; and (c) is primarily designed to provide an off-state gatevoltage on the CMOS devices that is above or below the supply voltagesby an amount equal to the saturation voltage of a CMOS device.

U.S. Pat. No. 7,760,007 discloses a low voltage analog CMOS switch. Thecircuitry described is designed to control the well voltage of the PMOSdevice to enable the switch to conduct properly for low voltages. It isdistinguished from the present invention as it does not disclose that itcan accommodate switch terminal voltages beyond the range of the supplyvoltages.

Application 2013/0194158 discloses a radio frequency switch for atransmitter/receiver IO port with electrostatic discharge protection. Itis distinguished from the present invention as it does not disclose thatthe switch can operate at terminal voltages that extend beyond thesupply rails.

Therefore, within the present invention is disclosed an apparatus andmethod for a bidirectional integrated CMOS switch that is novel in theart.

OBJECTS OF THE INVENTION

Accordingly, an object of the present invention is to provide abidirectional integrated CMOS switch capable of switching voltagesbeyond the range of the supply and ground potentials.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that can switch high voltage loads operating atarbitrary or floating voltages relative to the low voltage power supplyand ground.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that can enable on/off control of the switch withordinary low voltage levels.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that can operate with high terminal voltagesabove and below the supply rails.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that can provide the proper gate voltagesnecessary to operate the switch, regardless of the magnitude or polarityof the switch terminal voltages.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that can provide bidirectional switching withoutconducting through the parasitic body diode of the CMOS device.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that can be integrated on the same CMOS chip asother circuitry, eliminating the need for separate TRIAC switches,optocouplers, or other external devices.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that eliminates the need for charge pumps or highvoltage supplies.

Another object of the present invention is to provide a bidirectionalintegrated CMOS switch that is more cost effective and less prone tofailure than what is currently available in the industry.

SUMMARY OF THE INVENTION

The present invention combines three functional elements: (1) aNMOS/PMOS pair as the switched conduction path; (2) a diode bridge; and(3) control circuitry to turn the switch on and off using standard 5 vlogic (or other low voltage supplies that are compatible with the gatesource voltages needed to turn on the NMOS/PMOS pair), regardless of thevoltages on the switch terminals. The present invention makes use of ahigh voltage Silicon-on-Insulator (herein “SOI”) manufacturing processwith deep trench isolation, which provides circuit elements that areisolated from one another on the same silicon chip. The SOI processprovides the capability to create circuitry using diodes and MOSFETSthat reside in separate wells isolated from each other by non-conductivematerial, without parasitic substrate structures. An SOI process withdeep trench has non-conductive material under the bodies of the CMOSdevices and around the sides, so there are no diode connections to theP-substrate and there is no buried N layer underneath the P body of theNMOS. Except for the source-body and drain-body diodes, all of the otherparasitic diodes are eliminated. The diodes in the bridge are alsoisolated in individual wells, so there are no parasitic structuresassociated with them. The SOI process assures that the diodes andMOSFETs reside in separate wells isolated from each other bynon-conductive material, without parasitic substrate structures. Thepresent invention will also operate if a different process is used thatcan provide similar isolation capability,

The diode bridge allows current to flow in either direction between thetwo terminals of the switch without forward-biasing the parasitic bodydiodes of the CMOS switching elements.

The control circuit is configured such that the gate-source voltages ofthe isolated CMOS devices can be controlled regardless of the voltagesapplied to the switch terminals. The only limitation on the terminalvoltages is the maximum operating condition imposed by the processtechnology, which can be greater than +/−100V for some processes.

A novelty of the present invention is the circuit configuration thatenables CMOS circuit elements to switch any load within the limits ofthe process technology using ordinary low-voltage logic inputs ascontrol, without use of optocouplers, TRIACs, or other such componentsthat are not typically available in a CMOS process. This makes itpossible to integrate the switch on a chip with other analog and digitalcircuitry, thus eliminating the external switching components that aretypically used in power switching applications, such as motor controls.The present invention eliminates the need for charge pumps, boostercircuits and level shifters that are often used to provide the gatedrive for CMOS switches in high voltage applications, and insteadprovides gate voltage control independent of the magnitude or polarityof the switch terminal voltages relative to each other and relative tothe low voltage power supply rails.

In the present invention, the bodies of the PMOS and NMOS devices areconnected to their respective source nodes instead of the supply rails.Since there are no parasitic diodes to the substrate, the bodyconnections can be to any voltage that does not cause the body diodes tobecome forward biased. For each device, the source is connected to thebody so that the body voltage is equal to the source voltage and thesource-body diode cannot conduct. The diode bridge guarantees thatcurrent can only flow in a direction opposite to the direction thatwould turn on the drain-body diodes. Hence, the only possible conductionpath is through the CMOS device channel when it is turned on by applyingthe proper gate voltage. When the gate is biased so as to turn off thechannel, there are no conduction paths through the switch. There is onlya small amount of current to the source nodes due to the control networkstructures connected across the gate-to-source of each of the two CMOSswitch devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the invention's preferred embodiment.

FIG. 2 shows the use of the invention connecting a load to a floatinghigh voltage source of arbitrary polarity.

FIG. 3 shows an alternate embodiment of the use of the inventionconnecting a load to a floating high voltage source of arbitrarypolarity.

FIG. 4 shows the use of the invention connecting a load to anon-floating high voltage source of arbitrary polarity with an arbitraryoffset voltage.

FIG. 5 shows an alternate embodiment of use of the invention connectinga load to a non-floating high voltage source of arbitrary polarity withan arbitrary offset voltage.

FIG. 6 shows the use of the invention connecting a load between twoarbitrary voltage sources.

FIG. 7 shows an example of an alternative embodiment where the diodebridge is implemented as one component or a set of components separatefrom the rest of the components of the circuit.

FIG. 8 shows an example of an alternative embodiment in which the CMOSswitch elements corresponding to M0 or M1 are external to the integratedcircuit.

FIG. 9 shows an alternative embodiment in which M0, M1, D0, D1, D2 andD3 are implemented as components separate from the remainder of thecircuit.

FIG. 10 shows a further alternative embodiment in which diodes areplaced in the low voltage supply and ground paths.

FIG. 11 shows a still further embodiment in which switched currentsources are used to limit gate bias currents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an embodiment of the invention. All of the elementsin the diagram are high voltage isolated devices. Terminals A and B arethe switch terminals. The voltages at Terminals A and B may be anyvalue, positive or negative, within the limits of the processtechnology. Diodes D0, D1, D2 and D3 form a diode bridge which causescurrent to flow through the switch in only one direction, from Node C toNode D, regardless of the polarity of the voltages on terminals A and B.M0 and M1 are high voltage NMOS and PMOS transistors configured as atransmission gate, with parasitic body diodes oriented in the samedirection, opposing the direction of current flow through the switch.The diode bridge, M0 and M1 form the conduction path for current throughthe switch. Due to the diode bridge, the voltage at node C is alwaysgreater than or equal to the voltage at node D. Consequently, the bodydiodes of M0 and M1 are never forward biased.

PMOS device M2, diode D4, resistor R1 and the clamp connected from gateto source across NMOS device M0 form the primary ON/OFF control for M0.When the gate of M2 is pulled low, M2 is turned on and the current flowsthrough M2, D4, and R1 to pull up the gate of M0. The CLAMP limits thegate-to-source voltage of M0, protecting it from excessive voltage, evenwhen the source voltage at node D is a large negative value. Resistor R1limits the current to a low value to minimize bias current flow to nodeD. In an alternative embodiment, a switched current source may be usedto limit current. Diode D4 prevents current flow to the 5V supply whenthe voltage at node D is greater than 5V.

When the gate of M2 is pulled high, M2 is turned off. The gate-sourcevoltage of M0 is discharged through the CLAMP and the gate-sourcevoltage of M0 drops to zero, turning M0 off In the off state, devices M3and M5 are turned on to hold the gate-source voltage of M0 at zero, toprevent it from being turned on by charge that may be capacitivelycoupled to the gate from transients on the drain and source nodes.

Device M0 can be turned on when the voltage at node D is far enoughbelow the +5V supply rail to develop a forward bias on D4 and sufficientgate-source voltage is present on M0 to turn it on. In normal operation,M0 can be turned on whenever the voltage at node D is approximately+3.5V or less. Since the diode bridge causes the voltage at node C to begreater than or equal to the voltage at node D, turning on M0 willalways form a conduction path through the switch under these conditions.When the voltage at node D is too high to turn on M0, the conductionpath will be through M1.

NMOS device M7, diode D7, resistor R4 and the CLAMP connected from gateto source across PMOS device M1 form the primary ON/OFF control for M1.When the gate of M7 is pulled high, M7 is turned on and current flowsthrough M7, D7 and R4 to pull down the gate of M1. The CLAMP limits thegate-to-source voltage of M1, protecting it from excessive voltage, evenwhen the source voltage at node C is a large positive value. Resistor R4limits the current to a low value to minimize bias current flow to nodeC. In an alternative embodiment, a switched current source may be usedto limit current. Diode D7 prevents current flow to ground when thevoltage at node C is negative.

When the gate of M7 is pulled low, M7 is turned off. Thegate-source-voltage of M1 is discharged through the CLAMP and thegate-source-voltage of M1 drops to zero, turning M1 off. In the offstate, devices M8 and M10 are turned on to hold the gate-source voltageof M1 at zero, to prevent M1 from being turned on by charge that may becoupled to the gate from transients on the drain and source nodesthrough parasitic capacitance.

Device M1 can be turned on when the voltage at node C is far enoughabove ground to develop a forward bias on D7 and sufficient gate-sourcevoltage exists on M1 so that M1 can be turned on. In normal operation,M1 can be turned on whenever the voltage at node C is approximately+1.5V or greater. Since the diode bridge causes the voltage at node C tobe greater than or equal to the voltage at node D, turning on M1 willalways form a conduction path through the switch under these conditions.

For voltages between approximately +1.5V and +3.5V there is an overlapin the conduction conditions for M0 and M1 and both devices can beturned on. Together, devices M0 and M1 provide on-state conductionacross a large range of voltages on nodes C and D, from large negativeto large positive values, limited only by the maximum drain-sourcevoltage ratings of the devices. To conduct current through the switch,the absolute value of the voltage difference between terminals A and Bmust be sufficient to forward-bias the diodes in the diode bridge. AllPMOS and NMOS devices are isolated with the body connections shorted tothe source voltages. All of the CLAMP elements in FIG. 1 are used tolimit gate-source voltages and to provide a discharge path for gatecharge when the associated bias sources are turned off. A variety ofwell-known techniques may be used to construct the CLAMP elements, suchas circuits using series diodes, zener diodes, MOSFETs, resistors, orother devices available in the process technology. All of the resistorsare large value resistors for the purpose of limiting current. In otherembodiments of the present invention, any or all of the resistors may bereplaced by switched current sources.

The inverter (INV) shown in FIG. 1 may be replaced by a differentfunction such as controlling the NMOS and PMOS gates by a function otherthan direct inversion or to provide independent control of the NMOS andPMOS gates. In this embodiment nodes E and F could be interchanged sothat F is connected to the inverter input and E is connected to theinverter output. That would invert the input logic signal function suchthat H=OFF and L=ON.

Example 1 (FIG. 2) shows a circuit for connecting a load to a floatinghigh voltage source of arbitrary polarity. There is a low voltage on/offcontrol logic. The low voltage maybe about 5 volts. The switch of FIG. 1with the A and B terminals are connected to the floating high voltagesources of arbitrary polarity and a load as shown in FIG. 2.

Example 2 (FIG. 3) is similar to FIG. 2 except as to the location of theload. In FIG. 2 the load is between switch B and the floating highvoltage source of arbitrary polarity, while in FIG. 3 it is betweenswitch A and the floating high voltage source of arbitrary polarity.

In Example 3 (FIG. 4) the load is connected to a non-floating, highvoltage source of arbitrary polarity with an arbitrary offset voltage.As in FIGS. 2 and 3 there is a low voltage on/off control logicconnected to the circuit of FIG. 1. There is a low voltage supplytypically about 5 volts. The load is between switch B and both the highvoltage source of arbitrary polarity and an offset voltage of arbitrarypolarity.

In Example 4 (FIG. 5) the load is between the switch A and the highvoltage source of arbitrary polarity.

FIG. 6, Example 5, of the present invention has switch A connected to afirst high voltage source of arbitrary polarity. The load is connectedto switch B and a second source of high voltage of arbitrary polarity.

FIG. 7 shows an alternate embodiment of the present invention where thediode bridge is separate from the remainder of the CMOS circuit. In thisexample the diode bridge is implemented as a separate component or setof components from the remainder of the circuit. In this example nodesA, B, C and D correspond to nodes A, B, C & D in FIG. 1. Diodes D0, D1,D2 and D3 correspond to diodes D0, D1, D2 and D3 in FIG. 1.

FIG. 8 is an example of an embodiment in which M0 and M1 are implementedas discrete components separate from the rest of the circuitry.

An example of an embodiment in which M0, M1, D0, D1, D2 and D3 areimplemented as components separate from the remainder of the circuit isshown in FIG. 9. Nodes A, B, C and D correspond to nodes A, B, C and Din FIG. 1. M0, M1, D0, D1, D2 and D3 correspond to devices M0, M1, D0,D1, D2 and D3 as shown in FIG. 1, implemented as discrete components.

FIG. 10 is an example where the diodes have been removed from the biascurrent sources and instead placed in series with the 5V and groundsupply connections, either as integrated components on the chip or asexternal discrete components. In applications using a fabricationprocess that doesn't provide on-chip diodes, this approach could be usedin combination with a separate off-chip diode bridge. Since the purposeof the diodes in the bias current paths is to prevent reverse current tothe supply terminals, the result is the same whether they are at thesupply inputs or in the bias current sources. Likewise, it is possibleto move any of the circuit elements off-chip and substitute externaldiscrete components, as long as they are properly connected to the chip.This also applies to the embodiment shown in FIG. 9 above.

In addition, FIG. 10 is an example of an embodiment in which diodesrepresented by Dpos and Dss in the diagram are placed in the low voltagesupply and ground paths, either as integrated or discrete components,thus eliminating the need for diodes in the individual bias currentsources. In this embodiment the Dpos may be on-chip or off-chip, i.e.integrated or discrete. Similarly, Dss may also be on-chip or off-chip,i.e. integrated or discrete.

FIG. 11 is an example of an embodiment in which switched current sourcesare used to limit gate bias currents. In FIG. 11 the gate controlcurrents may be limited by means other than resistors, such as currentmirrors or other types of limited current sources. The gate controlcurrents may be supplied by switched current sources without the needfor current limiting resistors in these embodiments,

Although 5 volts is shown in the figures, one skilled in the art willappreciate that there can be embodiments in which the gate controloperates with a supply voltage other than 5 volts. There is standardlogic that uses 3.3 volts, for example, or a less common logic supplyvoltage may be used. The supply voltage does not matter, as long as theproper bias voltages can be generated to turn the switch on and off. Thepresent invention may also include embodiments where the nodes shown asground and +5V are connected to voltages other than ground and +5V, aslong as the voltage difference between the supply rails is of sufficientmagnitude and polarity to control the switch. Provided the connectedinput logic levels are consistent with the applied supply voltages, thecircuit would remain unchanged. The ground terminal does not necessarilyhave to be connected to zero volts. It is the difference between groundand the supply rail that matters.

In another embodiment one or more of the CMOS elements are replaced bybipolar transistors or other types of switching elements, such asopto-couplers, DMOS, JFETS, relays, etc.

What has been described is a novel bidirectional integrated CMOS switchcapable of switching voltages beyond the range of supply and groundpotentials. The application and embodiments described herein are givenas an examples of the useful nature of the invention and are notintended to limit the scope of the invention.

1. A bidirectional integrated CMOS switch comprising a NMOS/PMOS pair; adiode bridge; and control circuitry wherein the control circuitry turnsthe switch on and off using low voltage supplies that are compatiblewith gate source voltages needed to turn on the NMOS/PMOS pair andwherein the bodies of the control circuit elements are isolated from oneanother on a silicon chip by means of deep trench isolation usingnon-conductive materials to isolate circuit elements from one anotherand wherein said circuit elements reside in separate wells withoutparasitic substrate diode structures.
 2. The switch according to claim 1where said circuit elements includes one or more diodes and MOSFETS. 3.The switch according to claim 2 wherein the circuit elements that residein separate wells isolated from each other by non-conductive materialhave non-conductive material under the body of the circuit elements andaround the sides, so there are no diode connections to the P-substrateand there is no buried N layer underneath the P body of the NMOS.
 4. Theswitch according to claim 3 wherein diodes in the bridge are alsoisolated in individual wells, so there are no parasitic substrate diodestructures associated with them.
 5. The switch according to claim 4wherein the switch is formed by a high voltage Silicon-on-Insulator(herein “SOI”) manufacturing process.
 6. The switch according to claim 4wherein the diode bridge allows current to flow in either directionbetween two terminals of the switch without forward-biasing theparasitic body diodes of the CMOS switching elements.
 7. The switchaccording to claim 6 wherein the control circuit is configured such thatthe gate source voltages of the isolated control circuit elements arecontrolled regardless of the voltages applied to switch terminals. 8.The switch according to claim 7 wherein the bodies of the PMOS and NMOSdevices are connected to their respective source nodes instead of supplyrails.
 9. The switch according to claim 8 wherein there is a firstterminals and a second terminal which are the switch terminals andwherein the voltages at said first and second terminals may be positiveor negative, said switch further comprising a plurality of diodes thatform a diode bridge which causes current to flow through the switch inonly one direction, from a first Node to a second Node, regardless ofthe polarity of the voltages on the first and second terminals.
 10. Theswitch according to claim 9 further comprising first and second highvoltage NMOS and PMOS transistors configured as a transmission gate,with parasitic body diodes oriented in the same direction and opposingthe direction of current flow through the switch.
 11. The switchaccording to claim 10 wherein the diode bridge, the first and secondtransistors form a conduction path for current through the switch. 12.The switch according to claim 11 wherein the voltage at the first nodeis always greater than or equal to the voltage at the second node andbody diodes of first and second transistors are never forward biased.13. The switch according to claim 12 wherein the circuit connects a loadto a floating high voltage source of arbitrary polarity and the firstand second terminals are connected to said floating high voltage sourceand load
 14. The switch according to claim 13 wherein the load isbetween the second switch terminal and the floating high voltage sourceof arbitrary polarity
 15. The switch according to claim 13 wherein theload is between the first switch terminal and the floating high voltagesource of arbitrary polarity.
 16. The switch according to claim 11wherein the load is connected to a non-floating, high voltage source ofarbitrary polarity with an arbitrary offset voltage.
 17. The switchaccording to claim 11 wherein the load is between the second switchterminal and both the high voltage source of arbitrary polarity and anoffset voltage of arbitrary polarity.
 18. The switch according to claim11 wherein the load is between the first switch terminal and the highvoltage source of arbitrary polarity.
 19. The switch according to claim12 wherein the first switch terminal is connected to a first highvoltage source of arbitrary polarity and the load is connected to thesecond switch terminal and a second source of high voltage of arbitrarypolarity.
 20. The switch according to claim 12 wherein the diode bridgeis separate from the remainder of the CMOS circuit.
 21. The switchaccording to claim 12 wherein the first and second transistors arediscrete components separate from the rest of the circuitry.